Optical fiber is rapidly becoming the transmission medium of choice in many areas of telecommunications, including, for instance, long haul data and voice transmission. Essentially without exception, communications-grade optical fiber currently is silica-based fiber. As is well known, such fiber has a relative loss minimum at about 1.3 .mu.m, and an absolute minimum at about 1.55 .mu.m, and these are the wavelengths of currently greatest interest for optical communications purposes.
Even though silica-based optical fiber has reached a high degree of perfection, such that routinely achieved loss figures are close to the expected intrinsic loss of the material, a significant economic incentive still exists for development of lower loss optical fiber. For instance, fiber communications systems that are currently being installed typically have a repeater spacing of the order 20 to 40 km, which can in some cases be as high as 65 km, e.g., in an intercontinental submarine cable system that is soon to be installed. However, many applications exist where the distance between transmitter and receiver is of the order of 100 km to several hundred kilometers, and it would be highly desirable to have available low loss optical fiber which would permit repeaterless transmission over such distances.
Many glass systems have been identified which have substantially lower intrinsic loss than silica. Many of these contain heavy metals and have a refractive index greater than that of silica, but there also exist glasses that have lower intrinsic loss and lower refractive index than silica. Prominent among the latter are the fluoride glasses, and this application is principally concerned with optical fibers based on fluoride glasses containing BeF.sub.2 and having a refractive index that is smaller than that of SiO.sub.2.
The properties of fluoride glasses have been investigated in the past, primarily to determine their suitability for use in high power laser systems. From such investigations, it is known that the fluoride glasses tend to have relatively small linear and non-linear refractive indices, and frequently are transparent over a wide spectral region in the near to mid-infrared (see, for instance, K. H. Sun, Glass Technology, Volume 20(1), February 1979, pages 36-40).
Fluoride glasses are also known to have properties which make them potentially attractive for optical fibers. In particular, the intrinsic loss of these glasses is projected to be significantly less than that of silica, with minimum intrinsic losses of the order of 10.sup.-3 dB/km predicted from theory. Recently, results of research into possible designs of heavy metal fluoride glass optical fibers have been published. See, for instance, K. C. Byron, Electronics Letters, Vol. 18(15), pages 673-674 (1982), wherein it is reported that in such fibers the wavelength of zero total dispersion can be shifted over a relatively wide frequency range, that such fibers can have a relatively low slope of total dispersion as a function of wavelength, and that the total dispersion can be kept to a very small value over a relatively wide spectral region.
Attempts have also been made at the fabrication of heavy metal fluoride glass single mode optical fibers. For instance, Y. Ohishi et al, Journal of Lightwave Technology, Vol. LT-2(5), pages 593-596 (1984), report on the manufacture of single mode optical fiber using ZrF.sub.4 -based on fluoride glasses.
Heavy metal fluoride glasses typically have refractive indices greater than the refractive index of silica, and typically have minimum intrinsic loss at a wavelength larger than about 2 microns. Telecommunication systems using optical fiber made from heavy metal fluoride glass thus are expected to operate at wavelengths greater than about 2 microns, for which appropriate sources and detectors do not yet exist. On the other hand, BeF.sub.2 -based fluoride glass is predicted to have minimum loss at about 2 microns, and lower intrinsic loss than silica over the range 1.5-2 .mu.m. Since sources and detectors for these wavelengths either exist or can be produced by adapting existing technologies, communications systems that use BeF.sub.2 -based fluoride glass fiber as the transmission medium and that operate in the 1.5-2 .mu.m wavelength region offer advantages over both prior art SiO.sub.2 -based systems and proposed systems that use heavy metal fluoride glass fibers. However, such fibers can be expected to have different parameters (e.g., core radius a and core-cladding index difference .DELTA.) than prior art SiO.sub.2 -based fibers. It would thus be advantageous to have available design criteria and a process for designing BeF.sub.2 -based optical fibers having a predetermined wavelength of minimum dispersion .lambda..sub.o in the range 1.5-2.0 .mu.m. This application discloses such criteria and such a process. It also discloses BeF.sub.2 -based optical fibers that embody the criteria.